1. Field of the Invention
The present invention is directed to operating a probe-based instrument in torsional resonance mode, and more particularly, a method and apparatus of performing electrical property measurements using torsional resonance feedback.
2. Description of Related Art
Several probe-based instruments monitor the interaction between a cantilever-based probe and a sample to obtain information concerning one or more characteristics of the sample. Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically use a sharp tip to make a local measurement of one or more properties of a sample. More particularly, SPMs typically characterize the surfaces of such small-scale sample features by monitoring the interaction between the sample and the tip of the associated probe assembly. By providing relative scanning movement between the tip and the sample, surface characteristic data and other sample-dependent data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated. Note that “SPM” and the acronyms for the specific types of SPMs, may be used herein to refer to either the microscope apparatus, or the associated technique, e.g., “scanning probe microscopy.”
The atomic force microscope is a very popular type of SPM. The probe of the typical AFM includes a very small cantilever which is fixed to a support at its base and has a sharp probe tip attached to the opposite, free end. The probe tip is brought very near to or into direct or intermittent contact with a surface of the sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as an arrangement of strain gauges, capacitance sensors, etc.
Preferably, the probe is scanned over a surface using a high-resolution three axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other property of the sample as described, for example, in Hansma et al. supra; Elings et al. U.S. Pat. No. 5,226,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs can be designed to operate in a variety of modes, including contact mode and oscillating flexural mode. In contact mode operation, the microscope typically scans the tip across the surface of the sample while keeping the force of the tip on the surface of the sample generally constant by maintaining constant deflection of the cantilever. This effect is accomplished by moving either the sample or the probe assembly vertically to the surface of the sample in response to sensed deflection of the cantilever as the probe is scanned horizontally across the surface. In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography.
Alternatively, some AFMs can at least selectively operate in an oscillation “flexural mode” of operation in which the cantilever oscillates generally about a fixed end. One popular flexure mode of operation is the so-called TappingMode™ AFM operation (TappingMode™ is a trademark of the present assignee). In a TappingMode™ AFM, the tip is oscillated flexurally at or near a resonant frequency of the cantilever of the probe. When the tip is in intermittent or proximate contact with the sample surface, the oscillation amplitude is determined by tip/surface interactions. The amplitude or phase of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. As in contact mode, these feedback signals are then collected, stored, and used as data to characterize the sample.
Independent of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers typically fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
In the field of nanoscience and nanotechnology, it is extremely important to measure the electrical properties of various kinds of samples on the nanometer scale. Several techniques have been developed for this task. Among these techniques, scanning tunneling microscopy (STM), conductive AFM and scanning capacitance microscopy (SCM) are widely used. In STM, the sharp metal probe described above is brought close to a surface to be scanned, with a bias voltage applied between the tip and the surface. As known from quantum mechanics, there is some finite probability that electrons will tunnel through the insulating gap between the tip and the sample when the potential between the two is different, and the separation is small. This tunneling current is measured, and a feedback system changes the tip-surface distance to maintain a constant current at a set point as the tip is scanned across the sample. STM can be used to measure properties of metals, semiconductor and other materials with high to medium conductivities.
Notably, STM has a significant drawback. Since it uses the tunneling current as the feedback signal, the sample area being scanned needs to have some conductivity to allow the feedback loop to work throughout the scan. In general, STM cannot be used to scan an insulating sample or a conductive with insulating surface layers such as oxide. To overcome this problem, one known atomic force microscope, described in U.S. Pat. No. 5,874,734, uses a conductive probe with a sharp tip on a lever arm which is brought into contact with the surface of a sample to be scanned. The force of contact between the tip and the sample is measured by the deflection of the lever arm, with the feedback system moving the tip, or alternatively, the sample, up and down to maintain a constant force between the two during relative scanning motion produced by the AFM. During scanning, a constant or variable bias voltage may be applied between the tip and sample and the current distribution may be measured, preferably simultaneously.
The advantage of this technique is using the deflection force between probe and surface as the feedback signal to control the tip surface distance and force. The technique works on insulating samples with conductive patches and ultra-low conductivity samples. However, one drawback of this technique is that it uses the previously described contact mode of AFM operation, according to which a static vertical deflection force is utilized as the feedback signal to control the force, and thus the tip-surface distance, during scanning. There are several problems associated with this. First, the feedback can only maintain a constant force between the tip and the surface in the vertical direction. When the tip scans across the surface, there is generally a large shear force present, and this high lateral force can easily damage both the tip and the sample. Moreover, to run the microscope under stable imaging conditions, the tip and sample surface must remain in mechanical contact. This is a problem not only because of the high shear forces present in contact mode, but also because the measurement is useless if the tip and sample are not in contact. This issue has seriously limited the use of AFM-based electrical characterization techniques on soft samples like conductive polymers.
Next, the sensitivity of contact mode is limited because feedback is based on a static signal, as opposed to a dynamic signal. Static signals are more susceptible to thermal drift and charging, and thus sensitivity is compromised, as described in more detail below. For these reasons, contact mode is also not preferred when imaging soft and delicate samples.
These problems with contact mode operation led to the development of the previously described oscillating mode of AFM operation, see, e.g., U.S. Pat. No. 5,519,212. Again, in oscillating mode, a cantilever with a tip is driven to resonance at its flexural resonance frequency. The amplitude of the cantilever's flexural oscillations (between 20 nm and 100 nm) and the deflection angle of the cantilever (<100 nm) are detected by a quadrant photodetector, which outputs a voltage proportional to these two values. As the tip approaches a sample surface, the flexural oscillation (tapping) amplitude starts to decrease due to confinement of the surfaces of the tip and the sample. The flexural oscillation amplitude decreases to zero as the cantilever is lowered to the surface and therefore pushes the tip against the sample surface with an increasing contact pressure. Variation of amplitude between zero (contact) and free oscillation is used to control tip surface distance and force. Properties of the sample surface such a topography, hardness and electromagnetic properties can be acquired by raster scanning the tip over the sample surface, or vice versa, and controlling the tip/surface distance using the detected flexural oscillation amplitude.
Notably, in this regard, this oscillating mode feedback used to control tip-sample separation comprises dynamic signals. This is in contrast to contact mode which employs static signal feedback that reflects the absolute value of the acquired signal indicative of the motion of the cantilever at a certain point. With no reference, static signal feedback is susceptible to thermal drift and electrostatic charging, creating significant problems given that these phenomena directly affect the sensitivity of the measurement, as understood in the art. Active signals, on the other hand, reflect a relative shift in the acquired signals. By considering relative changes, dynamic signals are less affected by thermal drift and electrostatic charging. As a result, techniques that operate based on active signals are generally much more sensitive than those that rely on static signals, providing a significant advantage to oscillating mode.
Oscillating mode, as a dynamic measurement, benefits from a high “Q” value of the corresponding cantilever in air. The Q factor of a resonating cantilever is the width of the frequency response of the cantilever at half its maximum amplitude, divided by the resonance frequency of the cantilever. Notably, a higher Q factor in cantilever oscillation improves the signal-to-noise ratio in measurements that rely on variations in the amplitude and phase of cantilevers. The Q factor also reduces the effective force applied by the tip to the sample. As a result, TappingMode imaging is typically performed at much lower forces than contact mode, allowing routine imaging of much softer samples. Finally, in oscillating mode, the tip-surface contact time is a small fraction of the oscillation cycle, so the interaction force is mainly vertical and the shear force is dramatically reduced.
However, in oscillating mode, the cantilever resonant frequencies are generally greater than 10 kHz. Amplifiers sensitive enough to measure currents in the 60 fA range, which is of particular interest in the present case, are limited to bandwidths well below 1 kHz. Further, in oscillating mode as noted previously, the tip is in contact with the surface for a small fraction (for example 1%) of the oscillation cycle. Although this is a benefit for minimizing shear force, and thus sample damage, such minimal contact is a drawback in that the tip moves in and out of what is known as the “near field.” Because the tip of the probe must be in the near field to measure many electrical properties of the sample this often is a significant drawback with using oscillating mode to perform electrical measurements. Overall, for these reasons, it is generally not possible to perform low current measurements on samples while operating in oscillation feedback mode.
Therefore, one is most often left with operating the AFM in contact mode to perform these types of electrical measurements. As noted previously, in contact mode, there always must be mechanical contact between the tip and the sample surface, and thus a minimal force must always be applied. In fact, the tip must be caused to penetrate a water layer that resides on the sample surface and then to push into the sample surface; otherwise, stable imaging cannot be achieved. Therefore, a “soft” cantilever, i.e., one having a low spring constant, is typically employed in contact mode so as to minimize the magnitude of the forces applied to the sample surface, and thus minimize damage to the sample. In some cases, stiffer cantilevers are used to break through surface oxides and contaminants to make sound electrical contact.
Contact mode cantilevers often suffer from a serious drawback in that they cannot be brought into close proximity to the sample surface without the tip snapping into contact with the surface. This is due to the water layer on the sample surface that produces a meniscus force that acts on the lever as the tip approaches the sample surface. This is particularly a problem when performing force spectroscopy measurements, an illustration of which is discussed in connection with FIGS. 1–3.
When performing a spectroscopy or force measurement, probe-sample separation is controlled at a single scan location, i.e., X-Y location, as the deflection of the cantilever is monitored with an optical deflection detection scheme, for example. Typically, the tip is brought into contact with the sample surface at a certain speed and then withdrawn from the surface. As shown in FIG. 1, when a tip 22 of a probe 20 is brought towards a sample surface 24 (either by moving the sample towards a generally fixed probe, or moving the probe toward a generally fixed sample), the tip is “snapped” into contact with the sample surface from a relatively large separation distance, marked “A.”
This outcome is illustrated graphically in FIGS. 2 and 3 showing plots of distance (i.e., probe-sample separation) versus deflection. Initially, moving from right to left as shown by the arrows, the tip snaps to contact with the sample surface at about the point marked “P” as probe-sample separation is reduced. As a result, deflection of the probe downwardly increases. This snap-to-contact is illustrated more clearly in FIG. 3 at the region marked P showing (with the vertical portion of the graph) the cantilever instantly deflected downwardly as it is pulled by the meniscus force, and driving the tip into contact with the sample surface. The probe then deflects upwardly as the probe-sample separation is further decreased. Upon withdrawal of the probe from the sample surface in this spectroscopic measurement, the tip typically will adhere to the sample surface, again causing the cantilever to deflect downwardly until the point marked “Q” is reached, at which point the tip releases from the sample surface. This action between the probe and the sample yields regions of instability in the tip-sample separation continuum where the AFM cannot “hold” the tip to perform a current measurement. As such, in these regions, defined at I1 and I2, sample properties are generally “invisible” to the user.
The field of electrical property measurement was therefore in need of a system that enables nanometer scale measurement of ultra-low currents, for example (60 fA to 120 pA), correlated with topography on soft and delicate materials, and to acquire STM-type tunneling current data without relying on current as the feedback mechanism. The ideal solution would reduce tip wear and increase throughput for measurements of thickness and electrical properties of dielectric and insulating films. Preferably, the system would enable stable and localized measurements of I/V curves both in contact with the surface and with a small (i.e., nanometer scale) vertical offset from the surface. A system that allows the probe tip to remain in the near field (preferably, within a couple of nanometers of tip-sample separation), yet ensures that the probe does not snap into contact with the sample surface, would be ideal.